Batteries based on lithium (Li), such as lithium-ion batteries, are attractive due to their high energy density compared to other commercial batteries. Lithium-ion batteries are used commercially today in computers, cell phones, and related devices.
Lithium-based batteries (including lithium-ion, lithium-sulfur, and lithium-air systems) have significant potential in transportation applications, such as electric vehicles. For transportation-related applications, long cycle life is a requirement. Presently, this requirement has not been met.
Battery lifetime is often a critical factor in the marketplace, especially for commercial, military, and aerospace applications. Previous methods of extending battery life include employing long-life cathode and anode materials, and restricting battery operation to avoid conditions detrimental to battery life. Examples of such detrimental conditions include high and low temperatures, high depths of discharge, and high rates. These restrictions invariably lead to under-utilization of the battery, thus lowering its effective energy density. In addition, precise control of cell temperature with aggressive thermal management on the pack level is usually required, which adds significantly to system weight, volume, and cost.
A problem in the art associated with lithium-sulfur, lithium-air, and lithium-ion batteries is undesirable chemical migration that results in parasitic chemical reactions at the anode or cathode. For example, in batteries with manganese oxide and iron phosphate cathodes, dissolved metal ions often migrate to the anode where they are reduced and compromise the integrity of the solid electrolyte interface layer. Battery capacity degrades due to consumption of active ions. Battery storage and cycle life can be greatly improved if such undesirable chemical interactions are reduced or eliminated.
A successful battery separator layer should have a wide electrochemical stability window to be stable against the battery anode and cathode. In addition, the separator layer needs to have limited electronic conductivity in order to prevent electrical leakage between the two electrodes. When both requirements are imposed, the available materials are very limited.
It has proven difficult to maintain electrical isolation of the anode and cathode, while at the same time, provide lithium-ion conduction that will not limit the power performance of the battery cell. It would be beneficial to achieve thin-film solid-state ion conduction through a supported battery material layer on a porous polymer substrate. Such a configuration could potentially enable the use of electronically conductive materials, allow deposition of battery materials at low temperatures, relieve requirements relating to mechanical robustness, and contribute little (<50 mV) additional polarization.
Lithium-sulfur batteries have a theoretical energy density of 2500 Wh/kg. In practice, this energy density is not realized primarily due to an internal shorting mechanism. During discharge, when lithium and sulfur react, polysulfides are formed. These lithium-sulfur polymer compounds are soluble in the electrolyte and migrate in self-discharge from the cathode to the anode, creating a “soft” short within the cell. The impact of this soft short is a reduction of cycle life, energy density and cycling efficiency, as the polysulfides continue to build on the anode, and sulfur is lost from the cathode to the anode.
Prior approaches attempting to reduce polysulfide crossover in lithium-sulfur cells include cathode nanostructuring or encapsulation, electrolyte optimization (e.g., salt concentration or solvent composition), electrolyte additives (such as LiNO3) to protect lithium, and dual-phase or multilayer electrolytes. None of these approaches has the potential to completely eliminate self-discharge. What is needed is a cell configuration that can stop the crossover to, and deposition of, polysulfides formed during discharge on the anode. More generally, the configurations should prevent crossover of various lithium-containing compounds produced by lithium reactions (e.g., lithium polysulfides or other lithium-containing polymers, lithium dendrites, etc.).
The formation of lithium dendrites at the anode can also limit the cycle life of a lithium sulfur battery by driving up the cell resistance. Dendrites are formed during the charge cycle as lithium is deposited on the anode. If the current density is not uniform across the surface of the lithium anode, the lithium can be preferentially deposited in the areas with the highest current density. Deposition in these areas exacerbates the current density non-uniformity which propagates the formation of lithium dendrites on the anode. As the number of dendrites on the surface of the anode increases, the cell resistance increases, limiting the power performance of the cell. In addition, as the dendrites propagate, it is possible they will create a short in the cell as they move through the separator to the cathode. What is needed is a cell configuration that suppresses the formation of lithium dendrites during charge.
In view of the foregoing shortcomings, new battery cell structures are needed to address important commercialization issues associated with lithium-sulfur, lithium-air, and lithium-ion batteries.